Wednesday, December 24, 2014

The generation of gas resulting from the desorption is known as
outgassing. The outgassing constant is defined as the rate at which gas appears
to emanate from unit area of surface, and is usually measured in units of
Torr.Liter.Sec-1.Cm-2.

If the temperature of the material is raised (baking), the outgasssing
rate rises to a peak value.

Together with the acceleration of desorption, heating may also have the
effect of causing activated chemisorption of physically adsorbed gas (in
particular water vapour), which can then be desorbed only by prolonged heating
at much high temperatures.

Chemi-adsorbed water vapor continues to be evolved at temperatures in
excess of 300 degC. It should therefore appear that a degassing programme should
begin with pumping at room temperature to remove physically adsorbed water
vapor, before baking is commenced.

When
a material is placed in Vacuum, the gas which was previously adsorbed begins to
desorb i.e. to leave the material.

The
desorption is influenced by

1.Pressure

2.Temperature

3.Shape of material

4.Kind of its surface

The pressure has a basic influence on the desorption phenomenon since
according to its tendency of increasing over or decreasing below the
equilibrium , the phenomenon of sorption or that of desorption appears.

The temperature has a clear influence on desorption phenomena. Desorption
is endothermic, thus it is accelerated by increase of temperature.

The shape of the material influences desorption either if the gas is
adsorbed or absorbed.

If the gas is adsorbed, then only the amount of surface is the influencing
factor, but if the gas has to diffuse from the interior of the material to the
surface, then the third dimension “thickness” is also influencing the rate of
desorption.

They
provide more capacitance for their size than any other type. Electrolytics all
have one thing in common, instead of usual plates separated by a dielectric,
the electrolytic capacitor has a metallic anode coated with an oxide film. This
outer covering is the dielectric, and a liquid electrolyte acts as a cathode. A
second metallic conductor serves primarily as the connection to the liquid
cathode, providing an external termination.

In actual practice, porous paper is wrapped around the anode
and saturated with the electrolyte to eliminate the spillage problem.

There are two common types of electrolytic capacitors: a) Aluminum b)Tantalum

Both
employ the same basic principle. The aluminum or Tantalum anode is covered with
an oxide film. A suitable liquid or solid electrolyte is the cathode. The
Aluminum type is by far the most popular because of its lower cost. The
aluminum –oxide film has a very high resistance to current in one direction and
has very low resistance to current in opposite direction. In other words, film
acts as a dielectric in first instance and as a plate in second case. Because
of this electrolytic are polarized. If the designated polarity is not observed,
the oxide film on anode will breakdown and migrate to cathode connection,
resulting in prompt failure of capacitor.

Electrolytics
are described as belonging to one of the three basic families;

Polarized
type

This
type has one anode, the liquid cathode and a cathode connection. Polarity must
be observed.

Semi
polarized type

In
many energy storage applications a certain amount of current reversal is often
encountered. In such cases this type is recommended. In this type, the primary
anode has a relatively thick oxide surface. The cathode connection now becomes
the secondary anode with a thin oxide surface. Also liquid cathode has a
slightly different chemical composition.

Non-polarized

This
type is used in audio cross over networks and ac motor starting applications.
Here there is a complete reversal of polarity; therefore, two anodes are
required. The cathode connection now becomes a second, and equal, anode.
Obviously size will be affected. Infact, the third type called nonpolarized is just
twice as large as a polarized type of equivalent capacitance and voltage
rating.

Monday, December 22, 2014

Mica is popularly known as Isin glass. Important
characteristics of Mica:

i) Its ability to operate at very high temperature (upto 500
oC).

ii) The material is almost totally inert and will not change
with age, either chemically or physically.

iii) Mica is usable as a dielectric in its naturally state.

iv) It can be readily split into very thin sheets.

Mica capacitors are made by a method of depositing a thin
layer of silver on each side of sheet of Mica was developed. This is done by a
type of silver screen process. The silver is then fired in a furnace.

The principal advantage of Mica capacitor is its excellent
degree of stability over a wide range of operating temperatures.

Mica capacitors are also among the best types to use where
radio frequencies are involved.

Disadvantages of Mica capacitors:

i) They are relatively bulky when compared to other
capacitors on a pure capacitance vs volume basis.

ii) The flat plates and method of construction lead to
resonant frequency problems in some circuits.

Tuesday, August 19, 2014

A relay is an electromagnetic switch. An actuating current passed in a coil operates one or more galvanically separated contacts. In fact it is a remote controlled switch capable of switching multiple circuits, either individually, simultaneously or in sequence.

The most widely used type of relay is electromechanical relay. In electromechanical relays the switching element is a mecahncial contact, actuated by an electromagnet. This is the msot widely used type of relay design. The principal internal functions of the electromechanical relay are:

We are exposed
to radiation from the sun and outer space, also from the naturally occurring
radioactive materials present in the earth, the house we live in, the buildings
where we work, the food and drink we consume.

Where we live in

The
houses are made up of materials which contain radioactivity. Gamma radiation
from walls, floor & ceiling, and Radon and Thoron progeny are major sources
of radiation exposure. Especially in closed rooms, Radon is the significant
dose contributing factor.

Natural Radioactivity (Bq.Kg-1) in Building Materials used
in India

Material

Type
of Radioactive isotope

K-40

Ra-226

Th-232

Cement

5-385

16-377

8-78

Brick

130-1390

21-48

26-126

Stone

48-1479

6-155

5-412

Sand

5-1074

1-5047

4-2971

Granite

76-1380

4-98

103-240

Clay

6-477

7-1621

4-311

Fly
Ash

6-522

7-670

30-159

Lime
stone

6-518

1-26

1-33

Gypsum

70-807

7-807

1-152

We are always in natural background radiation*

Cosmic-from the sun and outer space – 0.4mSv.Yr-1

Terrestrial – from the earth’s crust – 0.5mSv.yr-1

Radon – from decay of Uranium/radium – 1.2mSv.yr-1

Internal sources in the body(eg: 40K)
--- 0.3mSv.yr-1

Total Dose from
Natural Sources: ----2.4 mSv.Yr-1

What we drink

Radioactivity in
milk is over 200 times more than in
drinking water and 3 times that in beer.

From a cup of milk ,
180 beta particles of Ptassium-40 are emitted per minute

From a cup of tea, 91
beta particles of Potassium-40 are emittd per minute from tea leaves(excluding
milk and water)

Monday, April 7, 2014

Cobalt-60 is a beta emitter that also releases gamma radiation. The energy released in the beta emission leaves the product element, nickel-60, in an excited state. When the nickel-60 descends to its ground state, it gives off photons in the gamma ray region of the radiant energy spectrum.